A well casing is typically placed in a well bore in order to protect the well bore from collapse and to facilitate the performance of various downhole operations. When the casing is positioned in the well bore, a cement slurry is typically pumped into the space (i.e., the annulus) existing between the wall of the well bore and the outer surface of the casing. Once hardened, the cement operates to prevent the cross-contamination of various underground formations through which the well bore extends, to protect the exterior of the casing from corrosive chemical attack, and to prevent well blowouts and fires which could otherwise be caused by the flow of high pressure gas through the annulus.
As is also known in the art, it is sometimes necessary to install a small diameter casing inside a large diameter casing. In a two-stage drilling operation, for example, a large diameter well bore is first drilled to a suitable depth using a large diameter drill bit. The large diameter well bore is then cased by (1) inserting a large diameter casing therein and (2) cementing the large diameter casing in place by pumping a cement slurry down the interior of the large diameter casing such that the cement slurry flows out of the lower end of the casing and then up into the annulus between the casing and the wall of the well bore. Next, a small diameter drill bit is lowered through the large diameter casing. The small diameter drill bit is used to drill a deep, small diameter well bore extending from the bottom of the large diameter well bore. The small diameter well bore is then cased by (1) inserting a small diameter casing therein such that the small diameter casing extends through the large diameter casing and into the small diameter well bore and (2) cementing the small diameter casing in place by pumping a cement slurry down the interior of the small diameter casing such that the cement slurry flows out of the lower end of the small diameter casing and then up into both (a) the annulus existing between the small diameter casing and the wall of the small diameter well bore and (b) the annulus existing between the small diameter casing and the interior surface of the large diameter casing.
In addition to cement, those skilled in the art will sometimes use one or more inflatable packers to seal the annulus between an inner casing and an outer casing. Gases, water, and/or other fluids can migrate through the cement column which fills the inner casing/outer casing annulus. Consequently, inflatable packers are also placed in the inner casing/outer casing annulus in order to halt the upward migration of these gases and fluids.
An inflatable packer suitable for incorporation in a well casing will typically be comprised of: an elongate tubular housing having a passageway extending longitudinally therethrough; means, provided at one end of the housing, for threadingly connecting the housing to the end of a first casing joint; means, provided at the other end of the housing, for threadingly connecting the housing to the end of a second casing joint; an inflatable packer element positioned around the exterior of the housing; and a housing port extending through the wall of the housing for filling the inflatable packer element with fluid (i.e., for inflating the packer element). When the inflatable packer is first inserted into the well, the packer housing port is temporarily closed by means of a knockoff pin which extends into the interior of the packer housing.
In order to seal the annulus between an inner casing and an outer casing using both cement and an inflatable packer, the inflatable packer is incorporated in the inner casing as the inner casing is inserted into the well. After the inner casing has been fully inserted into the well, a cement slurry slug is pumped into the inner casing. Next, a rubber plug is inserted into the inner casing. The rubber plug is pushed down the inner casing by pumping water, or some other fluid, into the inner casing. As the plug travels down the inner casing, the plug cleans the interior wall of the inner casing and forces the cement slurry slug out of the lower end of the inner casing into the inner casing/outer casing annulus. When the cement slurry travels up the inner casing/outer casing annulus, a portion of the slurry flows around the exterior of the inflatable packer so that the cement column placed in the inner casing/outer casing annulus extends from a point above the inflatable packer to a point below the inflatable packer.
As the rubber plug travels down the inner casing, the plug passes through the housing of the inflatable packer and thus removes the knockoff pin from the packer's housing port. The downward travel of the rubber plug through the inner casing eventually ends when the plug lands on top of a float shoe positioned at the bottom of the inner casing. When the rubber plug is positioned on top of the float shoe, the lower end of the inner casing is thus sealed such that, by continuing to pump fluid into the inner casing, the fluid pressure inside the inner casing can be greatly increased.
After the rubber plug lands on top of the float shoe and thus seals the lower end of the inner casing, the inflatable packing element of the packer is inflated by continuing to pump water, or some other fluid, into the inner casing. As the pumping operation continues, the fluid pressure inside the inner casing eventually increases to a point whereby fluid is forced through the packer housing port and into the packer's inflatable packing element. As fluid is forced into the inflatable packing element, the packing element expands (i.e., inflates) outwardly toward and against the interior surface of the outer casing. When the inflatable packing element is thus expanded against the interior surface of the outer casing, the inflated packing element seals the inner casing/outer casing annulus.